Enjoy the Kool-Aid
You will not regret the acquisition, I owned one for 15 years
and loved the bike dearly. If you are buying it from who I think you are ive seen that bike personally and its in great shape!
Its a tame yet snappy and fun engine.
The woods pipe will really help the low end.
its 220lbs dry, it will feel like a mountain bike compared to your KLR.
Check Fredette Racing for setup guides.
I use to mix in the tank on long rides, I remember riding about 50miles on a tank and never ran empty. I was just too exhaust to ride further.
I think 100 miles might be pusing it however that all depends on how much go juice you apply and what type of riding you are doing.
Upgrade the suspension and ride the bajesus out of it!
and then someday they will write a book about how good you can ride!